1. Field of the Invention
The present invention relates to Multi-Input Multi-Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) for communication using a plurality of antennas and a plurality of subcarriers, and also relates to the technique of a high-speed wireless LAN.
2. Description of the Related Art
In the conventional wireless LAN (802.11a), synchronous processing and channel estimation are performed by transmitting known symbols (a short preamble and long preamble) before a data field. By using these preambles, the subsequent signal and data can be demodulated. Recently, a high-speed wireless LAN standard called IEEE802.11n is being discussed. To achieve a transmission rate of 100 Mbps in a MAC layer, the IEEE802.11n is based on MIMO (Multi-Input Multi-Output) using a plurality of antennas. When the conventional preamble structures of this wireless LAN are to be applied to the MIMO system, therefore, the arrangements of a short preamble and long preamble must be changed to those for MIMO.
In a preamble structure proposed in a reference (Jan Boer and two other persons, “Backwards compatibility”, [online], September 2003, IEEE LMSC (publisher), [searched Sep. 15, 2003], Internet <URL: ftp://ieee.wireless@ftp.802wirelessworld.com/11/03/11-03-0714-00-000n-backwards-compatibility.ppt>), a short preamble sequence used for time synchronization, frequency synchronization, and automatic gain control (AGC), a long preamble containing a symbol for estimating a channel response, and a signal field are first transmitted from one transmitting antenna, and then long preambles for estimating channel responses are transmitted in turn from other transmitting antennas. After the transmission of the preamble is completed, data is simultaneously transmitted from a plurality of transmitting antennas. That is, long preambles for channel responses are transmitted from a plurality of transmitting antennas by time-division multiplexing.
In the MIMO system, the receiver must estimate the number of transmitting antennas in order to demodulate a transmission sequence. If this estimation of the number of transmitting antennas fails, the subsequent data field cannot be demodulated any longer. Therefore, the estimation requires very high accuracy. As a method by which the receiver estimates the number of transmitting antennas, it is possible to transmit a signal notifying the number of transmitting antennas from the transmitter. In this method, however, the overhead increases, and this unavoidably lowers the throughput of data transmission. It is also possible to estimate the number of transmitting antennas by using a received preamble signal. Since the preamble signal of the above reference is not for estimating the number of transmitting antennas, it is difficult to estimate the number of transmitting antennas by using this preamble.
In the MIMO system as described above, if the estimation of the number of transmitting antennas fails, the subsequent data portion cannot be demodulated any longer, so the estimation requires very high accuracy. On the other hand, the method by which the transmitter transmits a signal which notifies the number of transmitting antennas has the problem that the overhead inevitably increases. Also, in the technique described in the above reference, it is difficult to estimate the number of transmitting antennas by using the preamble signal.
It is, therefore, an object of the present invention to provide a wireless communication apparatus by which the receiving side can easily estimate the number of transmitting antennas used in transmission without any addition of a signal for notifying the number of transmitting antennas on the transmitter, and as a consequence a data symbol can be correctly demodulated.